Update: I contacted NVIDIA and reported the powermizer bug.
It is bug number 636716 in the internal NVIDIA bug tracking system,
so we can refer to it as such from now on.
GT240/GDDR5 (and probably other chips as well) are much slower under
Linux than under Windows, because powermizer only uses slow performance
levels, (e.g. 324MHz instead of the available 1.7GHz, where about 1/5th
of the memory bandwidth of the card is available).
As shown in the screenshot, the bug happens even while OpenGL or CUDA stuff
(reproducible bug with open source GPL CUDA code as well as
CUDA SDK samples)
(same bug reported for 260GTX and 280GTX cards)
Hoping that the next version of the Linux driver will fix this, so I can
proceed with my CUDA work...